West Nile is carried by Culex mosquitoes, which tend to bite at night.

West Nile is carried by Culex mosquitoes, which tend to bite at night.

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A neighborhood in North Dallas is sprayed for mosquitoes that carry the West Nile virus by a city of Dallas truck. Experts say the number of cases in Texas should continue to increase over the next few weeks.

Photo: Tom Fox

A neighborhood in North Dallas is sprayed for mosquitoes that carry...

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This is one way to keep mosquitoes off your face. (wetwebwork/Creative Commons Flickr)

This is one way to keep mosquitoes off your face....

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The attachment to a Beechcraft airplane that will disperse an insecticide to combat mosquitoes and an outbreak of West Nile virus, is displayed Thursday afternoon, Aug. 16, 2012, at Dallas Executive Airport in Dallas. For the first time in more than 45 years, the city and county planned Thursday to resume dropping insecticide from the air to combat the nation's worst outbreak of West Nile, which has killed 10 people and caused at least 200 others to fall ill. (AP Photo/The Dallas Morning News, Brad Loper) MANDATORY CREDIT TV OUT INTERNET: AP MEMBERS ONLY

Photo: Brad Loper, Associated Press

The attachment to a Beechcraft airplane that will disperse an...

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A Beechcraft airplane sprays the insecticide DUET over Dallas to curb the spread of West Nile virus Monday, Aug. 20, 2012. Storms interrupted aerial spraying on Thursday and Friday nights but county official said they were able to spray about 88,000 acres with pesticide that targets mosquitoes on Friday night. The remaining 222,000 acres were expected to be sprayed Sunday into early Monday morning. (AP Photo/LM Otero)

Lakey said state health officials are seeing signs that the mosquito-borne outbreak may have peaked in the northern part of Texas, the area by far the hardest hit. But he added that the number of cases and deaths in the state will keep rising at least into October.

439 cases in 2003

The state's previous worst year was 2003, when 439 people were diagnosed with West Nile disease. This year, 495 people already have been diagnosed with neuroinvasive West Nile, the severe form of the disease that affects the brain and can cause paralysis, coma and death. Neuroinvasive West Nile only affects 1 in 150 people infected with the virus, and most non-neuroinvasive cases are not reported.

Harris County has reported 38 cases, 22 of them neuroinvasive and three resulting in deaths. Nineteen cases have come in the last 10 days, the Houston area's peak period historically.

Petersen said Wednesday that the 1,993 cases of disease is the highest number reported to the CDC through the first week in September since the virus was first detected in the U.S. in 1999.

But the number would appear to have to continue to be reported at record levels the rest of the year to rival the nation's all-time high of 9,862, reported in 2003. The season typically lasts into October, though reporting can continue thereafter. The CDC numbers lag behind the latest state and local numbers.

The year's 1,993 cases represent an increase of 25 percent from the previous week. Petersen said that number is lower than last week's 40 percent increase because the Labor Day holiday shortened states' reporting period.

Spraying has helped

Three North Texas counties - Dallas, Tarrant and Denton - account for 719 of the state's cases, according to those counties' websites. Lakey said those numbers aren't increasing as quickly, however, an improvement he attributed to aerial spraying of insecticides.

Lakey said the state's analysis found the infected mosquito population in areas of Dallas sprayed on consecutive nights decreased 93 percent; the infected mosquito population in areas not sprayed increased 10 to 20 percent.

In Harris County, the mosquito population in areas sprayed by air in mid-August decreased 82 percent.

The state's 1,013 cases include 89 found in donated blood by people without symptoms.

About 80 percent of people infected with the virus develop no or few symptoms. One in five develops mild symptoms such as headache, joint pain, fever, skin rash and swollen lymph glands.

No vaccines exist to prevent West Nile infections, and no medications have been developed to treat it.

Infected mosquitoes spread the virus from birds to people, most commonly between June and October.